1. Field of the Invention
The invention relates to the field of ultrasonic imaging systems.
2. Prior Art
In recent years, more emphasis has been placed on the use of ultrasonics to provide anatomical images of soft tissue. This emphasis, to some extent, resulted from a public awareness of the dangers of X-rays, and also from the technical advances made in electronics. In the latter category, currently available integrated circuit memories, microprocessors, etc. have made possible the processing of signals that would not have been attempted a decade ago.
One of the more common imaging systems, commercially available today, employs a single transducer element which is manually moved over the area of interest to provide a compound B-display. The position of the transducer is followed through a relatively complex arrangement of arms and links to permit the painting of the B-display. This static imaging technique has the obvious disadvantage of not providing a real time display where movement of organs can be viewed. Moreover, even with a skilled operator, five to ten seconds are required for each picture, and through-put for such a system is generally one patient per hour.
A linear array of transducers is employed in other ultrasonic imaging systems. For example, 64 transducers are housed in a 15 centimeter long array in a commerical embodiment. These transducers are sequentially addressed to provide a B-display without movement of the array. The resulting single pass scans are of relatively poor quality since the smaller transducers required in the array have poorer characteristics than the larger transducers. These lower resolution images are used mainly in obstetrics where poorer images can be tolerated.
An imaging head with a single, larger transducer where the transducer is spaced-apart from the body is available. This imaging head is able to provide a wider field-of-view with a single transducer since the transducer is free to move. The transducer, which is housed in a bag of water brought into contact with the body, oscillates in an oscillating movement to provide the increased field-of-view. Placing the transducer away from the body to obtain this increased field-of-view introduces other problems. With the transducer spaced-apart from the body, reverberation between the transducer and the body may become significant. For example, if the transducer is spaced-apart from the body by a distance of three centimeters, the second reverberations sensed by the transducer may result in the appearance of an artifact at six centimeters below the skin. To solve this problem, the transducer is mounted a considerable distance from the body (e.g. 20 cm.). The resultant head is bulky and thus not easily handled. Another problem introduced by moving the transducer away from the body and having it in a water-filled bag (flexible membrane) is that the image is deteriorated because of refraction. The sound waves are disturbed as they enter and leave the body through the membrane. This cannot readily be corrected since the angle of incidence is unknown where a flexible bag, which conforms to the body in shape, is used.
More recently, an electronically steered ultrasonic array has been employed for ultrasonic imaging. Many transducers in the array are simultaneously addressed and each transducer is coupled through a variable delay line to provide an image. This array operates in a manner analagous to a phased array radar. However, these transducers are focused in one dimension only and often include undesirable side lobes.
As will be seen, the present invention provides an ultrasonic imaging head which has the advantages associated with the larger, high resolution transducers without the disadvantages inherent in the single transducer, static imaging devices. Those problems associated with the prior art heads where the transducers are spaced-apart from the body, are solved.